1. Field of the Invention
The invention relates to an onboard charging system and a control method thereof.
2. Description of Related Art
Japanese Patent Application Publication No. 5-111112 (JP 5-111112 A), for example, describes technology in which, in an electric vehicle provided with a main battery that supplies power for driving an electric motor for running (hereinafter, simply referred to as a “running electric motor”), and an auxiliary battery that supplies power for driving auxiliaries, the battery to which power generated by a solar cell is to be charged is switched according to the output power of the solar cell.
With the technology described in JP 5-111112 A, the voltage of power generated by the solar cell is stepped up to 150 V, for example, to make the voltage appropriate for driving the running electric motor, and this power is then charged to the main battery. Also, Japanese Patent Application Publication No. 2011-083072 (JP 2011-083072 A), for example, describes an onboard electrical system that steps down voltage using a DC-DC converter that is connected to an auxiliary battery, and then supplies this stepped-down voltage to an auxiliary battery.
When stepping up the voltage of power that has been generated by a solar cell using a step-up converter and charging this power to a battery, converter efficiency decreases and power loss increases as a step-up ratio of the step-up converter increases, so it is desirable to suppress an increase in this step-up ratio. With the related art described in JP 5-111112 A above, the battery to be charged (hereinafter referred to as the “charging target battery”) is selected according to the output power of the solar cell, so even if the output power is the same, the step-up ratio, and thus the power loss in the step-up converter, may increase.
For example, when comparing a first case in which the output power is 100 W, the voltage is 25 V, and the current is 4 A, with a second case in which the output power is 100 W, the voltage is 50 V, and the current is 2 A, when the voltage is stepped up to 200 V by a step-up converter, the step-up ratio in the first case is 8 and the step-up ratio in the second case is 2, so the step-up ratio, and thus the power loss, are higher in the first case.